On a surface of an optical disk such as a CD (compact disk) or a DVD (digital versatile disk), concaves and convexes called as pits are provided. An optical head device is a device for irradiating an optical disk with a laser beam and detecting light reflected from the disk to read information recorded in the pits.
For example, linearly polarized light emitted from a light source is transmitted through a beam splitter, a collimator lens, a retardation plate, and an objective lens to reach an information-recording plane of an optical disk. The linearly polarized light in the outgoing path is straightly transmitted through the beam splitter and transformed into circularly polarized light by the retardation plate. The circularly polarized light is reflected by the information-recording plane of the optical disk to be circularly polarized light in the reverse roll, and is then transmitted through the objective lens, the retardation plate, and the collimator lens in this order in the returning path in the reverse order to that of the outgoing path. The light is transformed by the retardation plate in the returning path into linearly polarized light polarized in a direction orthogonal to that before incidence. Accordingly, the light in the returning path, in which the linearly polarized light direction is 90° different from that of the light in the outgoing path, is shifted its traveling direction by 90° when the light passes through the beam splitter, and reaches a photodetector.
In the optical head device, if face deflection or the like occurs in the optical disk when reading or writing information, the focal position of a beam spot deviates from the recording surface. Accordingly, a servo mechanism for detecting such a deviation and shifting to make the beam spot follow the concave/convex pits on the recording surface is required. Such a mechanism is configured to adjust the focus of the beam spot emitted from a laser light source on the recording surface to detect a tracking position, and to make the beam spot follow an objective track. Moreover, in the optical head device, it is necessary to prevent a laser beam reflected by the recording surface without hitting the pits from directly returning to the light source.
For these reasons, the optical head device requires an optical element for modulating (polarizing, diffracting, phase-adjusting, etc.) the laser beam from the light source. For example, the above retardation plate has a function of imparting different refractive index to incident light depending on the angle between the optical axis of the retardation plate and the phase plane of the incident light, and shifting the phases of the two components of light produced by birefringence. The two light components having phases shifted from each other are synthesized when the light is output from the retardation plate. The magnitude of the shift of the phase is determined by the thickness of the retardation plate. Accordingly, by adjusting the thickness, a quarter wavelength plate for shifting the phase by π/2, a half wavelength plate for shifting the phase by π, etc. are produced. For example, linearly polarized light passed through a quarter wavelength plate becomes circularly polarized light, and linearly polarized light passed through a half wavelength plate becomes linearly polarized light having a polarization plane tilted by 90°. By utilizing such a characteristic and combining a plurality of optical elements, the above servo mechanism can be constructed. Moreover, the above optical element is employed also for preventing a laser beam reflected by the recording surface without hitting pits from directly returning to the light source.
The above optical element can be produced by employing a liquid crystal material. For example, a liquid crystal molecule having a polymerizable functional group has both of a characteristic of polymerizable monomer and a characteristic of liquid crystal. Accordingly, when such liquid crystal molecules each having a polymerizable functional group are aligned and then polymerized, an optical anisotropic material in which alignment of the liquid crystal molecules are fixed can be obtained. Since such an optical anisotropic material has an optical anisotropy such as a refractive index anisotropy derived from a mesogenic structure, by utilizing this characteristic, a diffraction element or a retardation plate is produced. As such an optical anisotropic material, for example, Patent Document 1 discloses an optical anisotropic material obtained by polymerizing a liquid crystalline composition containing a compound represented by CH2═CH—COO-Ph-OCO-Cy-Z (Z: alkyl group).